The invention relates to a toolholder for pin-type cutting inserts for machining, particularly copy turning and numerically controlled machining operations in a double-axed coordinate system.
Clamped cutting inserts have largely superseded brazed tools. Reasons for this development are the high expense involved in regrinding of the cutting edges, the difficulty of adhering to machining tolerance with tools which are not precisely reground and, above all, the increasing use of non-regrindable hard-coated inserts.
Two basic types of tool holders are used for clamped tools. In one case, the insert is embedded in a corresponding recess of the tool shank and is clamped from above by means of a clamp which rests on part of the upper surface of the insert. In the other basic design, a pin extends from the underside of the shank into the central bore of the pin-type tool and clamps the insert against the shank. According to one design, the insert is clamped through the bore by means of a screw against the tool shank.
In a clamped tool in accordance with the first basic design, the deeply embedded cutting insert can be exactly positioned in the tool shank and the compressive forces acting on the insert during turning can be normally positively transferred to the shank because the forces to be transmitted deviate only slightly from the ideal direction.
A cutting tool of this basic type, however, has a voluminously cantilevered tool shank and thus limits the directions from which the tool can be positioned in relation to the workpiece so greatly that various machining operations cannot be carried out. In addition to this serious disadvantage, there is always a danger in clamped tools that the chip cannot run off freely.
In contrast to this, a disposable insert with pin-type clamp is easily accessible and normally easily replaceable. Its exposed position on the end of the shank permits the unimpeded feeding of the insert which is particularly important in copy turning. On the other hand, the exact positioning of the insert presents problems. Furthermore, an adequate positive force transfer between insert with chip grooves and the shank is achieved only if, instead of the economic disposable insert with its small support surface, indexable inserts with cutting edges on one side only are used. Nevertheless, a satisfactory force transfer between workpiece and tool shank is frequently not obtained when forces from varying directions impinge on the cutting edge, as, for example, in turning in two coordination directions with alternating use of two cutting edges adjoining the nose of the tool.
The problem of climbing, i.e., lifting off the insert from the shank, and, consequently, chatter and vibration, is the reason why in a toolholder of the second type practically only prismatic and not positive rake inserts are used, although the latter have the advantage of allowing a greater variation of the angle between the insert and the workpiece. Even when prismatic inserts are used, the supporting surface on the shank must be inclined towards the working point of the tool in order to prevent climbing. However, due to the clearance angle required in turning, this measure shortens the usable length of the cutting edge and thus limits the possibility of turning with two cutting edges alternately, without indexing of the tool. The importance of this problem can be gauged from the fact that several makers of cutting inserts have deemed it necessary to develop expensive and not always satisfactory inserts with curved cutting edges.
Attempts have been made to combine the advantage of these two systems. For example, a triangular insert is placed on a shank with fixed pin and arrested by clamping a wedge at the side of the insert opposite the working point between the flank of the insert and the vertical shank surface. This type of clamping tool holder entails the risk of cutting edge damage through the wedge and also lacks the advantage of clamping of the insert from above. This applies even to the tools in which the wedge has a nose which rests on top of the insert because adequate froce transfer from above and from the side simultaneously through a single movable clamping component is very problematical.
In another toolholder design, the support of a pin tip by means of a pin and a clamp is achieved by a movable locking pin connected at right angles with a sliding bar which in turn moves in a nut in the shank below the supporting surface for the insert, ending below the threaded hole for the screw of the positioning clamp. By suitable facetting of the sliding bar blank and screw point, the screwing down of the clamp gives rise to a wedge action on the pin so that a thrust is exerted on the pin away from the tool point towards the wall of the bore in the insert. This toolholder design fulfills a number of tool requirements for turning, particularly copy turning. However, the exact positioning of the insert is not guaranteed. Furthermore, this design tends to undesirable wedging of the sliding bar in the shaft groove. This clamped tool holder was not very successful, however, mostly because the production is too expensive.
The design requirements for a clamping tool holder for copy turning are severe because, on the one hand, the inserts must be freely accessible to the workpiece, while on the other hand, the insert must be secured against slippage on the tool shank and precisely positioned.
Under these conditions, the use of several movable clamping elements makes possible an efficient transmission of the forces exerted on the insert from different directions of the workpiece. In practice, however, the fixation of the cutting inserts by the different clamping elements varies considerably in its effectiveness. In addition to design deficiencies, the insufficient ability of the clamping elements to slide relative to each other is a frequent source of malfunction.
Hence, it was the object of the present invention to reduce or resolve the above-described problems, particularly in connection with copy turning and machining in a double-axial coordinate system, through the new clamping tool design.
An additional object is to achieve a further increase in reliability of operation by a particularly advantageous design of the object of the invention.